This is a single-authored chapter in a book. The argument that I make in it is as follows. The default position taken by those working within this psychometric tradition of knowing other minds is that the individual has a number of capacities (i.e. knowledge sets, skills, dispositions), which we can describe as the contents of that person’s mind, and which subsequently we can characterise using the methods of experimentation and testing. There is thus potentially a true score for a person, and this true score represents in symbolic terms her capacity in the particular domain being tested. For a variety of reasons, errors may occur in the process of constructing that true score, but these are corrigible, i.e. they can be corrected by using different (and thus by implication better) methods and approaches. Errors may occur because the wrong type of instrument is chosen for determining the person’s true score or because that person’s emotional and psychological states are such that she gives a false impression of her capacities. However, there are a number of false assumptions being made here, and these underpin the PISA methodology.